1. Field of the Invention
This invention relates to a metallic gasket used to seal the joint surfaces of a cylinder head and a cylinder block of a multicylinder engine, and a method of manufacturing the same.
2. Description of the Prior Art
A metallic gasket formed out of a metallic material has been used to seal the opposed joint surfaces of a cylinder head and a cylinder block of an engine. In a recent engine, the increasing of the output level and the decreasing of the weight have been demanded. In order to meet these requirements, a cylinder head and a cylinder block tend to be formed out of an aluminum material having a low specific gravity instead of steel or a cast metal having a high specific gravity which has heretofore been used. An aluminum material has a small weight but the rigidity thereof is low. Therefore, while an engine in which the cylinder head and cylinder block are formed out of an aluminum material is operated, the relative displacement of the cylinder head with respect to the cylinder block tends to increase. A metallic gasket has beads near the circumferences of the through holes therein which correspond to the cylinder bores and passages for water and oil in these structural members of an engine. When the cylinder head and cylinder block are fixed to each other by tightening the same with bolts, these beads form annular resilient contact members with respect to the opposed joint surfaces of the cylinder head and cylinder block and seal the same joint surfaces.
The applicant of this invention has filed a patent application (Japanese Patent Application No. 306295/1990) concerning a single-plate metallic gasket to be disposed between the opposed joint surfaces of a cylinder head and a cylinder block of an engine and then tightened therewith, and a method of manufacturing the same. This metallic gasket has holes for combustion chamber formed in an elastic metallic plate, beads formed on the portions of the elastic metallic plate which are spaced radially outward from the circumferences of these holes, so as to extend along the same holes, and folded portions formed by bending the portions of the metallic plate which are around the holes, in the radially outward direction thereof onto the surface of the metallic plate on which the projecting surfaces of the beads extend, the folded portions being set to a predetermined thickness and heat treated. Accordingly, when the metallic gasket is tightened and pressed between the opposed joint surfaces of a cylinder head and a cylinder block, the beads form annular seals against these joint surfaces, and the folded portions the thickness of which is two times as large as that of the metallic plate form another seals extending around the holes. Thus, the beads and folded portions eliminate the irregularity of the opposed joint surfaces, minimize the flexure, which occurs due to the repetition of a combustion cycle, of the cylinder head, reduce fluctuating load stress and prevent the full compression of the beads.
The metallic gaskets disclosed in this publication include a metallic gasket in which a metallic plate the thickness of which is smaller than that of the elastic metallic base plate is laminated on the portions of the bead-ridge-carrying side of the base plate which are other than the portions thereof extending between adjacent holes, and which extend in the radially outward direction of the beads, so as to regulate a difference in thickness between the folded portions and the regions of the base plate which are on the radially outer side of the beads; a metallic gasket in which each of such folded portions is formed with a soft metallic plate held in a sandwiched state therein so that the difference in thickness referred to above does not become larger than a required one; a metallic gasket in which such folded portions are formed to a larger thickness on the regions between adjacent holes, and to a smaller thickness on the remaining regions so as to obtain a large pressing force on the former regions which demand high sealing performance; and a metallic gasket both surfaces of which are coated with a non-metallic gasket both surfaces of which are coated with a non-metallic material, such as heat and oil resisting rubber or resin so as to avoid the direct metal-to-metal contact between the gasket and the opposed joint surfaces of a cylinder head and a cylinder block, prevent the metallic surfaces from being corroded or soiled, and secure the excellent sealing function of the gasket even when the opposed joint surfaces of these parts of an engine are uneven.
The metallic gasket manufacturing method disclosed in the above-mentioned publication consists of the steps of forming holes for combustion chamber in an elastic metallic plate, molding beads on the portions of this metallic plate which are around and spaced radially outward from the circumferences of these holes so that the beads extend along the same circumferences, bending the portions of the metallic plate which are around the holes in the radially outward direction thereof onto the bead-ridge-carrying side of the metallic plate to form folded portions the edges of which are aligned with those of the holes, applying a compressive force to the folded portions so that the folded portions are formed to a predetermined thickness, and heat treating the metallic plate on which the folded portions have been formed so as to regulate the hardness of the plate. A metallic gasket manufacturing method identical with this method to which a step of inserting soft metallic plates in a sandwiched manner in the folded portions is added is also disclosed.
The applicant of the present invention has further filed a patent application (Japanese Patent Application No. 156189/1991), which is other than the above-mentioned prior-filed application, concerning a metallic gasket to be disposed and tightened between the opposed joint surfaces of a cylinder head and a cylinder block. This metallic gasket has holes for combustion chamber which are formed in an elastic metallic plate, beads formed on the portions of the metallic plate which are spaced from the circumferences of the holes in the radially outward direction thereof so as to extend along the same circumferences, and folded portions formed by bending the portions of the metallic plate which are around the holes in the radially outward direction onto the bead-ridge-carrying side of the metallic plate, in which gasket a regulating plate is laminated on the bead-ridge-carrying side of the metallic plate so as to extend in the radial direction of the beads, whereby the strength of the metallic plate is regulated.
In general, a metallic gasket consisting of a single elastic metallic plate employs an elastic metallic plate of 0.25 mm in thickness. When such an elastic metallic plate is bent at the portions thereof which are around the holes for combustion chamber to form folded portions, a difference in thickness corresponding to the thickness of the plate occurs between the portions thereof which are around the holes and the portions thereof which are on the radially outer side of the mentioned portions. It is considered that a desirable level of this difference is around 0.10-0.17 mm, though it depends upon the height of the beads. When this difference is too large, the portions of an elastic metallic plate which have such a difference in thickness are unduly deformed during a metallic gasket tightening operation carried out between the opposed joint surfaces of a cylinder head and a cylinder block. Since the portions thus deformed of the elastic metallic plate are bent, they necessarily become disadvantageous to resist stress, and further excessive stress would occur in these portions due to variation of load corresponding to a combustion cycle of the engine, to cause cracks to occur therein. The bead-carrying portions of such an elastic metallic plate serve as seals. When a metallic gasket having folded portions of a large thickness is tightened, strong seals are formed to prevent a high-temperature corrosive combustion gas from leaking from the holes for combustion chamber. Therefore, a load is applied one-sidedly to the folded portions around the holes, i.e., the requirements for excellent sealing performance are more concentrated on the folded portions than on the beads. Consequently, when the folded portions are broken while the gasket is used for a long period of time, the combustion gas leakage-preventing function thereof is not effectively fulfilled since the sealing force of the beads, which are provided so as to form two-stage seals with the folded portions, is not so large.
When a difference in thickness between the folded portion-carrying parts of the elastic metallic plate of this gasket is higher than a desired level, a plate thickness regulating plate is laminated on the elastic metallic plate so as to reduce the height of the folded portions. In such a thickness regulating method, it is necessary to provide regulating plates separately, so that the metallic gasket manufacturing cost increases. Moreover, in order to make regulating plates, it is necessary that the shape of the regulating plates be in agreement with that of the beads on the elastic metallic plate, and that the equipment for carrying out the laminating a regulating plate on the elastic metallic plate be provided. Therefore, this height difference regulating method is economically disadvantageous.
In the production of a metallic gasket having folded portions, the elastic metallic plate is subjected in some cases to so-called precompression in which the whole of the folded portions is compressed uniformly or a part thereof is compressed so as to set the thickness and width of different parts of the folded portions corresponding to different circumferential positions of the holes to different levels after the folded portions have been formed and before the gasket has been tightened between a cylinder head and a cylinder block. In such a method, the thickness of the plate is reduced after the folded portions have been formed, so that sudden and large variation in terms of engineering of materials occurs at the folded ends thereof. This would cause cracks to occur in the folded ends, or cracks to occur in the hole-side edges of the folded portions when the gasket is used for a long period of time with potential causes of cracks still retained therein.